In 1880, Charles Darwin and his son published a book, “The power of movement in plants”, in which they described plant tropic behavior. Through their detailed observations of the tropic curvature of monocot coleoptiles, they concluded that “when seedlings are freely exposed to a lateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend”. This observation is the first suggestion of the importance of “some influence” transmitted from the tip to basal growing parts, at least in the bending of monocot coleoptiles after lateral light stimulus. Following their suggestion, much research was conducted on plant tropic curvature, and indicated that the “influence” is a substance, auxin (indole-3-acetic acid; IAA), the first plant hormone to be identified. Tropic responses are generally explained by the Cholodny–Went Hypothesis; that is, they occur via differential growth on the two sides of the elongating shoot, which results from asymmetrical IAA distribution. However, their theory did not include the role of the apical tip region as the site of production of IAA, which is subsequently supplied to the lower growing region. A number of studies have shown the importance of the apical tip region of monocot coleoptiles and dicot seedlings for the perception of photo- and gravi-stimuli, as well as tropic curvature. However, the exact origin of the tip-localized IAA and the mechanisms regulating the amount of IAA remain unclear. In this mini-review, we summarize our research as a story from tip-specific IAA biosynthesis to gravitropic curvature in maize (Zea mays) coleoptiles.

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Elucidating the perception mechanism of auxin has been a long-standing goal in plant biology. To understand the mechanism, studies on the chemical structure and activity relationships of auxin-related small molecules have been done for more than half a century. Today, aided mainly by Arabidopsis genetics, researchers have identified the core complexes for auxin perception, including the auxin receptors TIR1/AFBs, SKP2 and ABP1. Forward chemical screening using auxin response markers has also played an important role in identifying small molecules affecting the auxin-signaling pathways and led to the screening of additional auxin related mutants. This approach has helped in identifying several unique molecular components of the auxin-signaling pathways, and discovering that would have been impossible using traditional screening of mutants against auxin. This review is focused on recently acquired knowledge of multiple auxin signaling pathways and related small compounds that can be used to dissect auxin signal transduction pathways. In addition, the involvement of distinct auxin-signaling machineries during gravity response is discussed.

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Etiolated plants show automorphosis (automorphogenesis), spontaneous growth responses under a stimulus-free environmental condition, under microgravity conditions in space as well as simulated microgravity (or weightlessness) ones on a 3-dimensional clinostat. Although the mechanism to regulate automorphosis has been unclear yet, in STS-95 space experiments a close relationship between automorphosis and reduced auxin polar transport in etiolated Alaska pea seedlings has been suggested. Induction of automorphosis-like epicotyl bending by auxin polar transport inhibitors strongly supports causal relationship of auxin polar transport to induce automorphosis. Genetic and molecular biological studies of Arabidopsis have demonstrated an importance of PIN-FORMED (PIN) proteins as carrier (or facilitator) subcellularly localized at the polar side of the plasma membrane for auxin polar transport. This review is describing automorphosis and auxin polar transport focused on structure and functions of PIN proteins together with hormonal regulation of automorphosis in etiolated Alaska pea seedlings.

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Phospho-glycoproteins (PGPs) belong to the ATP-binding cassette protein subfamily B (ABCB) subgroup of the ATP-binding cassette (ABC) transporter superfamily. The involvement of ABCB proteins in auxin transport is first suggested base on the results of expression levels of PGP1/ABCB1. PGPs have also been shown to mediate the cellular and long-distance transport of auxin. Interactions among PIN-formed and P-glycoprotein in auxin transport have been shown although PINs and PGPs characterize coordinated and independent auxin transport mechanisms. ABC (ABCB)/PGP is also suggested to have an important role in a graviresponse in plants. This review is describing auxin transport and a graviresponse in plants focused on structures and functions of ABC proteins.

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Bone is a dynamic tissue that is constantly remodeling. However, for individuals requiring bed rest or astronauts exposed to microgravity in space, a lack of normal loaded activities leads to disuse osteoporosis. We examined whether knee loading increases bone mineral density (BMD) of both control and tail-suspended hindlimbs. We also examined whether the phosphorylation levels of Akt in phosphoinositide 3-kinase (PI3K) signaling and eukaryotic initiation factor 2α (eIF2α) in an integrated stress response (ISR) is altered in the loaded femur and tibia. Knee loading was applied to the right hindlimb using a custom-made piezoelectric mechanical loader. The BMD of control and tail-suspended hindlimbs was determined by PIXImus imaging, and Western blot analysis was performed to determine the phosphorylation levels of Akt and eIF2α. The results revealed that knee loading significantly increased the BMD in the loaded limb of tail-suspended mice. Compared to non-loaded limbs in the hindlimb-suspended mice, loaded limbs demonstrated a 6.6% increase in BMD. In the control group, the loaded limb showed a 7.4% increase in BMD over the non-loaded limb. Furthermore, knee loading activated PI3K signaling through the induction of Akt phosphorylation and reduced ISR by the suppression of eIF2α phosphorylation. The present study thus supports knee loading as a potential loading method of preventing the detrimental effects of disuse due to unloading.

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To investigate whether terrestrial life (TL) can survive in interstellar and interplanetary space, an experiment was performed to simulate conditions in the Earth's orbit. There are many factors which influence survivability. Among them, the effects of temperature fluctuations and UV irradiations were addressed in this paper. Six species of moss spores and 3 species of fungal spores were selected as target TL. Temperature was fluctuated between 80˚C and -80˚C once every 90 min, whereas UV at 254nm was irradiated for 10 min (471mJ/cm²) and 30 min (1,413mJ/cm²). The moss spores of Funaria hygrometrica (exp 1) and Pogonatum inflexum (exp 2) were the most tolerable species to the thermal cycle treatment, with germination rates of 3.3±1.2% (n=5) and 7.9±3.2% (n=6), respectively, after 3 weeks of treatment. Germination occurred even after the spores had been UV irradiated for 30 min (0.7% to 23.5%). Only a slight difference in the germination rate was observed using different culture media. The treated spores were transferred to soil where they grew into gametophytes, then sporophytes, and finally formed new capsules after 7-8 months. Two species of fungal spores were allowed to adsorb onto beads before the beads were directly irradiated for 10 min and 30 min, respectively. Colonies developed the spores irradiated for 30 min. On the other hand, a colony did not develop if the spores were taken off the beads and irradiated for 10 min. This indicates that UV does not penetrate to the other side of the beads, and so the spores on that side can be protected from UV radiation.

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The differences between two types of Prunus sp., a Japanese cherry tree, were the first investigated by an acoustic vibration analysis. The resonance frequency by the acoustic vibration was very different among the different types of tree forms, i.e, upright and weeping. Two Types of trunk and basal part of branch were analyzed. In all of the tested parts, the sound speed of the trunks and branches in the weeping type of tree showed a tendency to be faster than that in the upright type of tree. The uniformity of composition in their wood was also estimated by the coefficient-of-variation (CV) of the sound speed. The CV of the weeping type of tree was lower than that of the upright type of tree. This result for the weeping type in both its trunks and branches indicates that the weeping type of tree would have a uniform composition. Based on this result, the function of the tension wood formation in the two types of tree was discussed. Our results suggest that this analysis would be useful for determining the inside composition including the tension wood formation as a nondestructive method after being exposed to outer space environment.

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